An article by Nicholas Riccardi in Friday’s Los Angeles Times cites global climate change as the primary cause of Sudden Aspen Decline, which has been sweeping through forests of the American West in recent years. Rising temperatures and increased drought conditions—both attributed in part to global warming—have increased populations of insects, such as the aspen bark beetle and aspen leaf miner, to which aspens are highly vulnerable. SAD has ravaged aspen groves in Colorado and elsewhere, significantly transforming the landscape.

Whereas these forests are declining due to the effects of climate change, forests globally have the potential to reduce climate change. Through photosynthesis, healthy forests contribute oxygen to the atmosphere and remove carbon dioxide by storing carbon in plant tissue. This lowers the concentration of atmospheric carbon dioxide. Decadent or dormant forests, however, release carbon. As aspen groves die, they stop taking up carbon and release the carbon they had stored.

Forests, of course, can also affect climate locally and regionally by releasing moisture. Tree roots withdraw water from the soil for transport up the stem or trunk to the leaves where the moisture evaporates. The increased atmospheric humidity can reduce or prevent drought.Trees can also extract moisture from the air by their contact with low fog, causing it to condense on leaf surfaces and drip to the ground, where it can add substantially to total annual precipitation. Trees also moderate local temperature extremes and wind velocities.

“In addition to their influence on [local and global] climate, forests purify water by filtering it through litter and soil. Much of the water we drink, either form surface or underground sources, comes from forested watersheds, including water that accumulated eons ago. Forests also increase the amount of water reaching groundwater reservoirs by slowing the rate of surface runoff (which helps prevent floods), thus increasing the percolation of runoff in to the soil. This helps recharge deep groundwater, raises the water table, and makes for more persistent streamflow during dry seasons, benefiting vegetation and wildlife . . . Soil and forest litter absorb rain like a sponge and release it to vegetation and groundwater slowly . . . More than half of the water supplies in the western United States flow from national forests.”2

Aspen trees, like most forests, provide a rich habitat for many different plant and animal species. The grasses that sprout under aspen groves help slow runoff, hold soil, reduce erosion, and encourage infiltration of water into the ground, which is important for making water available to nearby metropolitan areas. Therefore, the decline of the aspen in the American West means not only a loss of scenic beauty, ecological vitality, and municipal water supply, but is a harbinger of the pervasive forest loss that climate change will bring to much of the American West and Southwest.